This invention relates to a process for converting a waxy hydrocarbon feedstock into a high quality product hydrocarbon having a reduced concentration of normal paraffins. It is particularly concerned with a process for producing a lube oil base stock having a relatively low pour point or a middle distillate having either a low freeze point or both a low pour point and low cloud point from a full boiling range shale oil.
Many liquid hydrocarbon feedstocks contain relatively high concentrations of straight chain and slightly branched chain aliphatic compounds having between 8 and 40 carbon atoms. These long chain compounds tend to crystallize on cooling of the hydrocarbon oil. This crystallization is quite frequently sufficient to hinder the flow of the liquid hydrocarbon and prevent it from being pumped or transmitted from one location to another. The temperature at which the hydrocarbon oil will not flow is commonly referred to as the "pour point" and is determined by standardized test procedures. One such feedstock having a relatively high pour point is the raw oil obtained by retorting oil shale, such as the oil shale found in the Colorado River formation in the western United States.
Oil shale is a mixture of a minor amount of solid organic matter known as kerogen and a major amount of mineral matter. Hydrocarbons are normally recovered from oil shale by subjecting it to heat via pyrolysis or retorting at temperatures between about 850.degree. F. and about 1000.degree. F. These high temperatures cause the kerogen to decompose into liquid and light gaseous hydrocarbonaceous products. The liquids recovered by condensing the oil shale vapors will normally contain a relatively high concentration of straight and slightly branched chain paraffins of high molecular weight. This high concentration of waxy components typically results in the oil having a relatively high pour point, normally between about 50.degree. F. and about 90.degree. F. In addition, the raw shale oil will contain arsenic, organonitrogen constituents and/or organosulfur constituents.
U.S. Pat. No. 4,153,540 teaches a method of upgrading raw shale oil by removing the organonitrogen and organosulfur compounds and also reducing the pour point. The shale oil is treated in a two-step process in which the shale oil is first contacted with a hydrotreating catalyst under conditions such that the concentration of organosulfur and organonitrogen constituents is reduced. The hydrotreated shale oil is then contacted with a catalyst under hydrocracking conditions in the presence of molecular hydrogen such that the feedstock is hydrodewaxed to some extent while at least 50 percent of its 750.degree. F.+boiling fraction is converted by hydrocracking to products boiling below 750.degree. F. The catalyst utilized comprises a ZSM-5 zeolite in its hydrogen form combined with a metal having activity for promoting hydrogenation/dehydrogenation reactions. The use of ZSM-5 and related porous, crystalline aluminosilicates results in the conversion of some of the straight and slightly branched chain paraffins into lower boiling components, thereby decreasing the pour point of the treated oil.
The process described in U.S. Pat. No. 4,153,540 has a serious disadvantage if it is desired to convert the raw shale oil into a lube oil base stock or middle distillate of relatively low pour point. Since lube oil base stocks normally boil between about 650.degree. F. and about 1000.degree. F., it is undesirable in treating the shale oil to convert a large portion of its higher boiling constituents to lower molecular weight constituents which boil in the gasoline range. Similarly, since middle distillates normally boil between about 300.degree. F. and 700.degree. F., it is undesirable to convert a large portion of the shale oil's higher boiling constituents into lower molecular weight constituents that boil below about 300.degree. F. It appears, however, that the hydroconversion step disclosed in the process of U.S. Pat. No. 4,153,540 is quite nonselective in that not only are waxy paraffins hydrocracked to lower the pour point but 50 percent or more of the 750.degree. F.+constituents are cracked as well. Such excess hydrocracking results in substantial yield losses when the desired product is a high quality lube oil base stock having a relatively low pour point or a middle distillate having either a low freeze point or both a low pour point and low cloud point.
In order to avoid excessive yield losses in the process disclosed in U.S. Pat. No. 4,153,540, the catalytic conversion step can be carried out with the same catalyst at lower severity conditions. It has been found, however, that although significant yield losses are avoided by this technique, the pour point, cloud point or freeze point of the products are not sufficiently decreased. Evidently, under more mild conditions, the catalyst is very selective to cracking of the straight chain paraffins while leaving a large proportion of the slightly branched chain paraffins in the oil.
Accordingly, it is one of the objects of the present invention to provide a process for converting a raw shale oil or other waxy hydrocarbon feedstock into (1) a lube oil base stock having a relatively low pour point without substantially decreasing the yield of lube oil base stock constituents boiling in the 650.degree. F.+range or, alternatively, into (2) a middle distillate having either a relatively low freeze point or both a relatively low pour point and cloud point without substantially decreasing the yield of middle distillate constituents boiling in the range between about 300.degree. F. and 700.degree. F. It is another object of the invention to provide such a process having the further advantage of selectively hydrocracking the straight chain and slightly branched chain paraffins while not substantially hydrocracking other components to low boiling products.